Autophagy is a cellular homeostatic mechanism with broad roles in human health and disease (Mizushima et al., 2008). Autophagy is at the intersection of metabolic (Rabinowitz and White, 2010; Settembre and Ballabio, 2014) and antimicrobial processes (Deretic et al., 2013; Ma et al., 2013). Thus, the system responds to a range of inputs such as starvation (Chauhan et al., 2013; Efeyan et al., 2013; Mihaylova and Shaw, 2011), lysosomal disruption (Settembre and Ballabio, 2014), endogenous danger associated molecular patterns and microbial products commonly referred to as pathogen-associated molecular patterns (PAMPS) (Deretic et al., 2013; Ma et al., 2013). Autophagic responses to PAMPS lead to direct antimicrobial action through a process termed xenophagy (Gomes and Dikic, 2014; Levine, 2005) and control of inflammation and other immune processes (Deretic et al., 2013).
Among the better-established links between autophagy and human diseases are the genetic polymorphisms in ATG16L1 and IRGM conferring risk for Crohn's disease (CD), an intestinal inflammatory disorder (Consortium, 2007; Craddock et al., 2010; Murthy et al., 2014). The human population polymorphisms in IRGM have been linked to autophagy (Consortium, 2007; Craddock et al., 2010) and to its effector outputs including antimicrobial defense (Brest et al., 2011; McCarroll et al., 2008). In keeping with its autophagy-mediated antimicrobial role, IRGM is additionally a genetic risk factor for tuberculosis in different human populations (Bahari et al., 2012; Che et al., 2010; Intemann et al., 2009; King et al., 2011; Song et al., 2014) and may afford protection in leprosy (Yang et al., 2014). However, the molecular mechanism of IRGM's function in autophagy has remained a mystery.
IRGM has no homologs among the Atg genes in yeast, which makes it difficult to assign to it an autophagy-specific function; instead, IRGM has been considered to affect autophagy indirectly (Singh et al., 2006). A complicating factor in understanding the exact function of IRGM is that it is distinctly a human gene (Bekpen et al., 2010). Its orthologs are present only in African great apes and Homo sapiens but active alleles are absent in ancestral evolutionary lineages leading up to them (Bekpen et al., 2009). The mouse genome encodes a large family of immunity related GTPase (21 IRG genes) compared to a single gene (IRGM) in humans; furthermore, all murine IRGs encode ca. 40-kDa proteins that are much larger then the human IRGM (21 kDa). The prevailing view of the murine IRGs is that they have predominantly non-autophagy functions (Choi et al., 2014; Zhao et al., 2008). Thus the significant information gathered in the murine systems may have limited import on how the human IRGM works.
Given the significance of IRGM in human populations and the notoriously high prevalence of diseases such as CD and tuberculosis, it is surprising that IRGM's mechanism of action in autophagy remains unknown. Here we report that unexpectedly, IRGM physically interacts with key autophagy regulators, ULK1, Beclin 1, ATG14L and ATG16L1. We also show that, remarkably, IRGM links inputs from PAMP sensors by making molecular complexes with NOD2, another genetic risk factor in CD (Eckmann and Karin, 2005; Hugot et al., 2001; Ogura et al., 2001). The formation of NOD2-IRGM complex is stimulated in response to PAMPs, whereas increased association of NOD2 with IRGM promotes IRGMdirected assembly of autophagy regulators. IRGM undergoes post-translational modifications that stabilize components of the core autophagic machinery, and mutant IRGM protein that cannot direct these modifications is disabled for its role in autophagic defense against invasive bacteria.
Therapies to modulate autophagy are entering clinical trials but methods of monitoring whether drugs modulate autophagy in patients during such treatment are currently unavailable, but badly needed. In one aspect, the present invention addresses that need.